The determination of sample solids fraction or sample moisture content is a routine laboratory determination. Many agricultural products, food products, and manufactured products (e.g., textiles, films, coatings, paper, and paints) are sold based on solids content or moisture content. Consequently, solids and moisture analyses are frequently run. Unfortunately, monitoring moisture and solids content using conventional techniques is exceedingly time consuming. For example, drying a sample in a convection oven to achieve solids content takes upwards of four hours and typically requires desiccation of the dried sample.
There are, however, high-speed analytical procedures for volatilizing moisture or solvents to facilitate quantitative analysis of various substances (e.g., agricultural commodities, foodstuffs, dairy products, chemicals, paper products, and tobacco). These procedures often employ microwave energy to heat a material sample to remove various volatiles. Thereafter, moisture, solids, or other residuals and losses can be determined. To achieve these weight measurements rapidly and accurately, the sample is not removed from the balance, but rather weighed in place after each succeeding step. Such automation reduces the possibility of human error.
The weight of the substance is often sensed or measured repeatedly during the microwave heating while volatiles are being removed from the heated sample. Consequently, such methods not only require sensitive analytical balances, but the capability to measure weight while the sample is hot. These methods, however, fail to correct weight measurements for the buoyancy effects that are caused by temperature and pressure variances within the microwave cavity. This failure can introduce significant errors to determinations of solids or moisture content.
U.S. Pat. No.4,753,889 (hereinafter the Collins '889 patent), which is commonly assigned with this application, is directed to rapid quantitative analyses of materials having high moisture content. The Collins '889 patent discloses the evaporation of moisture to determine the solids and other materials present without removing or destroying the other materials when removing moisture. In particular, rapid analysis is facilitated by microwave heating to drive off moisture, followed by solvent extraction, and content determinations. U.S. Pat. No. 4,753,889 is hereby entirely incorporated by reference.
Similarly, U.S. Pat. No. 4,291,775 (hereinafter the Collins '775 patent), which is commonly assigned with this application, addresses the problem of disruptive convection currents. More specifically, the Collins '775 patent describes a method and apparatus for improving the weighing accuracy of sensitive automatic balances when weighing heated substances by introducing an air barrier shield to cover the balance plate without contacting or touching the automatic balance. This tends to reduce the convection currents that can interfere with the sensitive balance and, consequently, hinder the achievement of accurate sample weight measurements. In other words, the eliminating convection air currents reduces movement of the balance and thereby fluctuations in the measurement of sample weight being sensed. U.S. Pat. No. 4,291,775 is hereby entirely incorporated by reference.
Finally, U.S. patent application Ser. No. 09/397,825 (Microwave Apparatus and Method for Achieving Accurate Weight Measurements), which is commonly assigned with this application, improves upon the teachings of the Collins '775 patent by including an air shield that is removably secured to the inside of the heating cavity such that the air shield does not contact an analytical balance when the air shield is fastened to the heating cavity. By securing the air shield to the interior of the microwave cavity, a laboratory technician or instrument operator need not manipulate a movable barrier that must rest on the cavity floor, yet be placed so that the barrier substantially surrounds both the analytical balance and the sample to be heated. U.S. patent application Ser. No. 09/397,825 (Microwave Apparatus and Method for Achieving Accurate Weight Measurements) is likewise entirely incorporated by reference.
While these methods overcome specific problems associated with microwave heating and drying, none discloses nor teaches a method for correcting apparent sample weight to account for buoyancy effects caused by different air densities surrounding a sample. Therefore, there is a need for a method to correct weight measurements for the buoyancy effects that are caused by temperature and pressure variances within a heating environment.